In a vibrating beam accelerometer, a proof mass is suspended from a support by a flexure hinge, and a vibrating beam force transducer is connected along the sensitive axis of the accelerometer between the proof mass and the support. An acceleration along the sensitive axis results in a compression or tension force on the vibrating beam, resulting in a change in the beam vibration frequency. The beam vibration frequency is measured and used to determine acceleration.
A push-pull vibrating beam accelerometer typically comprises two vibrating beam sensors, as described above, mounted in a common housing such that a given acceleration results in a compression force on one vibrating beam and a tension force on the other vibrating beam. Each sensor produces an output signal having a frequency corresponding to the sensed acceleration, and the acceleration is calculated essentially as a function of the difference in the frequencies of the two output signals. This push-pull approach is typically more accurate than a single sensor instrument, due to the elimination of common mode errors.
Prior vibrating beam accelerometers have demonstrated the accuracy required for many kinds of inertial navigation and missile guidance applications, but have also been characterized by certain limitations. One limitation results from the fact that a vibrating beam acceleration sensor is an inherently non-linear device, and the sensor output must be linearized in some fashion before acceleration is integrated to determine velocity change. Another limitation is that output circuits used with vibrating beam accelerometers cannot measure velocity changes that occur during time intervals in which electrical power is unavailable. Such accelerometers are therefore incapable of operating through a short-term loss of power. Two prior art techniques exist for dealing with the first limitation, but they are complex and expensive. No techniques have previously been available for measuring velocity change over a time interval in which a power loss occurred.
Both electronic and mechanical techniques have previously been available for dealing with the nonlinearity of vibrating beam accelerometers. The electronic approach consists of measuring frequency by counting cycles of the sensor output signals (or signal) against a common clock, dividing the output frequency counts by the clock count to determine the frequencies, and then sampling and numerically processing the result to obtain acceleration. The acceleration is then numerically integrated to determine velocity change. This process is complex, and the sampling must be done at high speed to achieve accuracy. The mechanical approach to the nonlinearity problem is specific to the dual sensor accelerometer, and consists of mechanically trimming and matching the zero acceleration vibration frequencies and scale factors of the two sensors, so that a simple frequency difference is a sufficiently linear measure of acceleration. However the mechanical adjustments required to match two sensors to a few parts per million are difficult and timing consuming, and tend to be self-defeating since they must be made prior to sealing the sensors and isolating the sensors from the environment. The mechanical trims and adjustments can therefore never be fully stabilized.